The physiologic activities of recirculating lymphocytes are studied in murine animals in an effort to understand further the function of these cells in immune responsiveness. Experiments are performed on the relationship between lymphocyte surface structures and the distribution of the cells in the body. Emphasis will be placed on examining the mechanism of lymphocyte homing to high-endothelial venules in lymph nodes. This will be studied using an in vitro assay which permits direct observation in lymphocyte interaction with the specialized venules in fixed sections of lymph nodes.